Radiometric determination of anthropogenic radionuclides in seawater

Status of the transfer state of 137Cs in zooplankton and surface water fish off Fukushima during 2018-2021

Radiocaesium introduced to coastal waters by the accident at the Fukushima Dai-ichi Nuclear Power Station (F1NPS) elevated the radioactivity level in biota. The radionuclide level in zooplankton, concentration of 137Cs radioactivity in surface water fish decreased rapidly, which was the primary food source for planktivorous fish, was recognized as not to be depurated at the same level prior to the accident. To evaluate the possible cause of this phenomenon, zooplankton and surface water fish were collected off Fukushima during 2018-2021, and the presence of radioactive particles was also examined. The concentrations of stable Cs and 137Cs radioactivity were analysed along with aluminium which was an indicator element of Cs that originated from suspended particles which were attached to or ingested by zooplankton. As a result, radioactive particles were often found in coastal zooplankton samples, and stable Cs and 137Cs of this inorganic fraction were identified. After removal of this excess radioactivity, the 137Cs radioactivity in whole-body tissue of zooplankton was derived. However, the level in the soft part of zooplankton during 2018-2020 was still greater than the levels which existed before 2010. Since habitat seawater was understood to not be a substantial source of 137Cs, then 137Cs transfer along the food chain, possibly from phytoplankton or detritus of enhanced radioactivity were suggested as important sources of 137Cs. In addition, enhanced 137Cs radioactivity in zooplankton was considered consequently elevate radioactivity levels in surface-dwelling water fish off Fukushima. Although the radioactivity level was not radiologically significant in relation to seafood safety limit, enhanced 137Cs radioactivity levels in biota was demonstrated in southern waters off Fukushima. In contrast, derived 137Cs/133Cs atom ratios of fish and seawater south of 37°E and west from 142°E indicated that radiocaesium transfer between fish and the environment was in an equilibrium state, showing the environment beyond these geographical coordinates had returned to the pre-accident state.

Radioecological behaviour of 137Cs in rockfish of the southern coastal waters off Fukushima during 2017-2021

Following the accident at the Fukushima Dai-ichi Nuclear Power Station, radiocaesium concentrations were specifically elevated in rockfish species compared to other fish species. To clarify the likely reasons, a caesium metabolic rate in the Japanese rockfish Sebastes cheni was derived by an aquarium experiment of live fish collected from the area off Fukushima. Stable caesium and 137Cs concentration in prey organisms, stomach contents and muscle of rockfish were measured and the bioavailable fraction in prey organisms was evaluated. Using derived transfer parameters, 137Cs radioactivity levels in S. cheni and prey organisms were simulated by a model, and verified by the measured radioactivity concentrations of biota in coastal waters south of the Fukushima Dai-ichi Nuclear Power Station. As a result, slow caesium metabolism in S. cheni was confirmed with the biological half-life (Tb1/2) of 190 d. The determining factor for the initial 137Cs radioactivity levels in S. cheni, was the maximum radioactivity levels in surrounding seawater which was constrained by the sedentary nature of rockfish. Controlling factors of depuration rate of 137Cs levels in S. cheni were slow caesium metabolism, enhanced 137Cs radioactivity level of prey organisms, and survival of older contaminated individuals due to a long life-span. During the study period 2017-2021, 137Cs radioactivity concentrations in seawater decreased close to the level measured before 2010, whereas those in prey organisms and rockfish in southern Fukushima waters were still above the levels that existed before 2010. An additional source for enhancing 137Cs radioactivity in rockfish and biota of the food chain was indicated by the greater 137Cs/133Cs atom ratios in rockfish compared to those in the surrounding seawater, however it was considered to be radiologically insignificant in relation to seafood safety limits.

Dissolved Organic Carbon Source Attribution in the Changjiang Outflow Region of the East China Sea

The variable optical properties of chromophoric dissolved organic matter (CDOM) under the complicated dynamic marine environment make it difficult to establish a robust inversion algorithm for quantifying the dissolved organic carbon (DOC). To better understand the main factors affecting the relationship between the DOC and the CDOM when the Changjiang diluted water (CDW) interacts with the marine currents on the wide continental shelf, we measured the DOC concentration, the absorption, and the fluorescence spectra of the CDOM along the main axis and the northern boundary of the CDW. The sources of DOC and their impacts on the relationship between the optical properties of the DOC and CDOM are discussed. We reached the following conclusions: There are strong positive correlations between the absorptive and fluorescent properties of the DOC and the CDOM as a whole. The dilution of the terrestrial DOC carried by the CDW through mixing with saline sea water is the dominant mechanism controlling the characteristics of the optical properties of the CDOM. CDOM optical properties can be adopted to establish inversion models in retrieving DOC in Changjiang River Estuary. It is concluded that the introduction of extra DOC from different sources is the main factor causing the regional optical complexity leading to the bias of DOC estimation rather than removal mechanism. As whole, the input of polluted water from Huangpujiang River with abnormally high a(355) and Fs(355) will induce the overestimation of DOC. In the main axis of CDW, the impact from autochthonous DOC input to the correlation between DOC and CDOM can be neglected in comparison with conservative dilution procedure. The relationship between the DOC and the CDOM on the northern boundary of the CDW is more complicated, which can be attributed to the continuous input of terrestrial material from the Old Huanghe Delta by the Subei Coastal Current, the input of materials from the Yellow sea by the Yellow Sea Warm Western Coastal Current, and the input of materials from the Changjiang Basin by the CDW. The results of this study suggest that long-term observations of the regional variations in the DOM inputs from multiple sources in the interior of the CDW are essential, which is conducive to assess the degree of impact to the DOC estimation through the CDOM in the East China Sea.

Behavior of Radiocesium in Sediments in Fukushima Coastal Waters: Verification of Desorption Potential through Pore Water

Concentrations of ¹³⁷Cs in seawater, seabed sediment, and pore water collected from the area around Fukushima were investigated from 2015 to 2018, and the potential of coastal sediments to supply radiocesium to the bottom environment was evaluated. The ¹³⁷Cs concentration in the pore water ranged from 33 to 1934 mBq L^-1 and was 10-40 times higher than that in the overlying water (seawater overlying within 30 cm on the seabed). At most stations, the ¹³⁷Cs concentrations in the overlying water and the pore water were approximately proportional to those in the sediment. The conditional partition coefficient between pore water and sediment was [0.9-14] × 10² L kg^-1, independent of the year of sampling. These results indicated that an equilibrium of ¹³⁷Cs between pore water and sediment has been established in a relatively short period, and ¹³⁷Cs in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that ¹³⁷Cs in the sediment decreased by about 6% per year by desorption/diffusion of ¹³⁷Cs from the seabed.

Impacts of direct release and river discharge on oceanic 137Cs derived from the Fukushima Dai-ichi Nuclear Power Plant accident

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. We used the Regional Ocean Model System (ROMS) to simulate the ¹³⁷Cs activity in the oceanic area off Fukushima, with the sources of radioactivity being direct release, atmospheric deposition, river discharge, and inflow across the domain boundary. The direct release rate of ¹³⁷Cs after the accident until the end of 2016 was estimated by comparing simulated results with measured ¹³⁷Cs activities adjacent to the 1F NPP. River discharge rates of ¹³⁷Cs were estimated by multiplying simulated river flow rates by the dissolved ¹³⁷Cs activities, which were estimated by an empirical function. Inflow of ¹³⁷Cs across the domain boundary was set according to the results of a North Pacific Ocean model. Because the spatiotemporal variability of ¹³⁷Cs activity was large, the simulated results were compared with the annual averaged observed ¹³⁷Cs activity distribution. Normalized annual averaged ¹³⁷Cs activity distributions in the regional ocean were similar for each year from 2013 to 2016. This result suggests that the annual averaged distribution is predictable. Simulated ¹³⁷Cs activity attributable to direct release was in good agreement with measurement data from the coastal zone adjacent to the 1F NPP. Comparison of the simulated results with measured activity in the offshore area indicated that the simulation slightly underestimated the activity attributable to inflow across the domain boundary. This result suggests that recirculation of subducted ¹³⁷Cs to the surface layer was underestimated by the North Pacific model. During the study period, the effect of river discharge on oceanic ¹³⁷Cs activity was small compared to the effect of directly released ¹³⁷Cs.

The detection of Fukushima-derived radiocesium in the Bering Sea and Arctic Ocean six years after the nuclear accident

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, radionuclides released by this event were observed in the Pacific Ocean. Models predicted that these radionuclides would be transported to the Bering Sea; however, limited evidence currently reveals the transportation of these radionuclides to the Arctic Ocean. Here, we provide the first direct observation showing that FDNPP-derived ¹³⁴Cs and ¹³⁷Cs were present in subarctic regions and the Arctic Ocean (Chukchi Sea) in 2017. Furthermore, we conclude that these radionuclides were transported from the Pacific Ocean into the Bering and Chukchi Seas by ocean currents. Additionally, the ¹³⁷Cs activity concentrations in the Bering Sea exceed those in all previous reports. Due to the continuous leaking of radionuclides from the FDNPP, we hypothesize that FDNPP-derived radionuclides will be continuously transported to the Arctic Ocean in the next several years. Our results suggest that though far away from Fukushima, the accident-derived anthropogenic radionuclides also influenced the Arctic Ocean by ocean currents.

Temporal Variation of Radionuclide Contamination of Marine Plants on the Fukushima Coast after the East Japan Nuclear Disaster

As a result of the Great East Japan Earthquake and associated tsunami in March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) released a large amount of radioactive material into the environment, resulting in contamination of many marine organisms. In this study, 15 marine algal species and a seagrass species were collected from the sublittoral zone of the Iwaki Coast of Fukushima Prefecture from May 2012 to June 2015 and analyzed for variations in ^110mAg, ¹³⁴Cs, and ¹³⁷Cs over time. The results indicated that (1) ^110mAg, ¹³⁴Cs, and ¹³⁷Cs were present in all marine plants collected in May 2012, (2) the concentration of ^110mAg in the seagrass Phyllospadix iwatensis decreased significantly over time while the ecological half-life of ^110mAg in P. iwatensis was longer at locations closer to the FDNPP, and (3) the ^110mAg/¹³⁷Cs radioactivity ratio of P. iwatensis was remarkably high until 2015, indicating that detectable ^110mAg was present in the coastal environment 4 years after the accident. The concentration of ^110mAg in P. iwatensis was higher than those in other marine algae, demonstrating a species-specific mechanism of accumulation.

An integrated device for coprecipitation and filtration of radiocesium in seawater

To improve the pretreatment efficiency of radiocesium in seawater, a prototype machine based on a new design of an integrated device for coprecipitation and filtration was developed and tested in the laboratory for its possibility and efficiency, the results show the that the efficiency of pretreatment of radiocesium can be improved more than one orders of magnitude compared to traditional methods. And the results from experiments on effect of settle time show that immediately filtration of solution after reaction will not affect the absorption of AMP to radiocesium in seawater.

Excess of (236)U in the northwest Mediterranean Sea

In this work, we present first (236)U results in the northwestern Mediterranean. (236)U is studied in a seawater column sampled at DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (Ligurian Sea, 43°25'N, 07°52'E). The obtained (236)U/(238)U atom ratios in the dissolved phase, ranging from about 2×10(-9) at 100m depth to about 1.5×10(-9) at 2350m depth, indicate that anthropogenic (236)U dominates the whole seawater column. The corresponding deep-water column inventory (12.6ng/m(2) or 32.1×10(12) atoms/m(2)) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5ng/m(2) or 13×10(12) atoms/m(2)), evidencing the influence of local or regional (236)U sources in the western Mediterranean basin. On the other hand, the input of (236)U associated to Saharan dust outbreaks is evaluated. An additional (236)U annual deposition of about 0.2pg/m(2) based on the study of atmospheric particles collected in Monaco during different Saharan dust intrusions is estimated. The obtained results in the corresponding suspended solids collected at DYFAMED station indicate that about 64% of that (236)U stays in solution in seawater. Overall, this source accounts for about 0.1% of the (236)U inventory excess observed at DYFAMED station. The influence of the so-called Chernobyl fallout and the radioactive effluents produced by the different nuclear installations allocated to the Mediterranean basin, might explain the inventory gap, however, further studies are necessary to come to a conclusion about its origin.

Southwest intrusion of 134Cs and 137Cs derived from the Fukushima Dai-ichi nuclear power plant accident in the Western North Pacific

Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived (134)Cs (1.5-6.8 Bq m(-3)) was 18 °N, 135 °E, in September 2012. The potential density at the subsurface peaks of (134)Cs (100-500 m) and the increased water column inventories of (137)Cs between 0 and 500 m after the winter of 2011-2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of (134)Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the (134)Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPP-derived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water.

Southward spreading of the Fukushima-derived radiocesium across the Kuroshio Extension in the North Pacific

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected ¹³⁴Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived ¹³⁴Cs in the North Pacific Ocean.